The suggestion to use angle iron was only intended for the 900mm
opening on an upstairs window, anything much larger and the brickwork
will have to be supported using needles,props etc. How old is your
house, as it was built with cavities it's suprising there are no
lintles on the outer leaf, are the windows showing signs of bowing?
personally i would get professional help installing a lintel for a 2.3m
opening.
i used 2" x 1/4" angle iron which did the trick , i also wedged it in
at both ends to prevent it falling out.